The invention relates to a process for the production and installation of pressed-stock boards.
Such a process with an installation is suitable for the production of pressed-stock boards such as gypsum boards, gypsum fiberboards, cement fiberboards, boards of wood-based material such as particle boards, fiberboards and chipboards, as well as for the production of boards comprising a combined component of wood and plastic or composed of plastic with and without reinforcing inclusions.
The process and the installation are described by taking as a representative example the producing of gypsum fiberboards, it being self-evident that in the production of the pressed-stock boards enumerated above differences especially in the design of the prepress and of the main press with respect to the introduction of heat and the exertion of pressure depend on the respective pressed stock.
Generally, in the continuous production of single-layer or multi-layer gypsum fiberboards, the individual layers are scattered from a scattering station onto a molding and charging belt. During the scattering operation, the pressed stock slips away laterally, causing, at the edges of the scattered mat of pressed stock, an inclined edge of the loose fill with an angle of repose of about 45 xc2x0, as illustrated in FIG. 8. This region becomes unusable due to the inadequate scattered weight, since the raw board density required in the pressing operation is not achieved. In the production of 16 mm boards, this effect makes it necessary for the edges to be trimmed by 50 mm on each side. With a customary finished board width of 2,440 mm, this already represents a material loss of 4%. In a further process step, the scattered mat of pressed stock is compacted, initially in a prepress and subsequently in a main press, in order to drive out the air accumulated in the loose fill.
In the production of 16 mm boards, the scattering height is about 80 to 100 mm. The scattered mat of pressed stock is compacted in the prepress to an exiting height of about 18 to 25 mm. In the first part of the main press, the remaining compaction of the board to nominal size then takes place. During this pressing of the mat of pressed stock, horizontally acting forces are released within the mat of pressed stock, causing the scattered material to be squeezed out to the sides and leading to a widening of the mat of pressed stock. The frictional forces present at the surfaces in contact with the upper and lower pressing belts oppose the horizontal forces and reduce the increase in width. This results in a bulging, as in FIG. 9 shows. The material which is displaced outward originates only from the border region of the mat of pressed stock. Further in within the mat of pressed stock, a sufficiently large opposing force has formed, so that the latter is in equilibrium with the horizontal forces and no material displacements take place in the horizontal direction. The displacement of the material from the border region has the consequence that not only the bulging even after the pressing operation has to be trimmed, but so too does the entire region of displacement, in which there is no longer sufficient material to achieve the required apparent density. In the production of 16 mm boards, trimming of a further 50 mm per edge is required as a result. Consequently, altogether 8% of the originally scattered material has to be trimmed. At the same time, it is only with difficulty that the trimmed material can be fed back to the production process, since the gypsum has already set and may lead to the formation of lumps during recirculation. This results not only in costs due to the loss of material, but also in costs for the disposal or reprocessing of the waste.
In the production process according to European Patent 0 590 095, the water necessary for hydration is added only after the scattering of the individual layers. After the addition of water, the pressed stock gains a very high force of adhesion.
Tests already carried out to support the scattered mat of pressed stock within the scattering station with fixed side walls and also to guide it through the prepress, in a way corresponding to already known scattering walls, for example according to German Patent 576 346 in the production of wood fiberboards, have failed. The fibers and the gypsum stick to fixed side walls and material builds up to form layers of relatively great thickness, which within just a few hours make the installation inoperative.
It is known in the case of installations for the production of gypsum fiberboards according to the wet method, in which excess water is used and squeezed out during the pressing operation, to provide a side limitation by way of a circulating belt of foam material composed of cellular Vulkollan. In the wet method, however, there are only low compaction ratios of about 1:2, since the pressed stock is already of a high density.
In the production of gypsum fiberboards by the dry method according to European Patent 0 590 095, there is a compaction ratio of 1:4 to 1:7. A foam material which serves as a scattering wall in the region of the scattering station and subsequently runs along with the belt through the prepress and main press in order to act there as a side limitation must likewise withstand this compaction ratio. It has been found in practice that such soft foam materials have only very short service lives. Gypsum crystals and perlite get into the pores of the foam material and destroy it by their abrasive action. Moreover, they are so soft that, under the horizontal forces occurring during compaction of the mat of pressed stock, they move away outward and consequently only prevent squeezing out on a limited basis.
A further problem of flexible foam materials is that the foam material immediately reverts to its original shape on leaving the press after the pressing operation. Since it is firmly interlocked with the product due to the horizontal forces acting in the press, when the foam material breathes the outside edge of the product is pulled up with it and delamination of the edge region is the consequence.
A further disadvantage of a flexible edge limitation is that the foam material hermetically seals the space between the upper and lower transporting belts and consequently the accumulated air in the pressed stock can no longer escape to the sides. The air can then only escape forward, counter to the direction of production, or downward through the metal mesh belt.
An object of the present invention is an installation having a process for effectively limiting the width of the mat of pressed stock from the scattering station through the prepress and, if appropriate, also through the main press.
Another object of the present invention is to ensure satisfactory side limitation of the pressed stock or of the mat of pressed stock up to when the finished board leaves the prepress or the main press.
A further object of the present invention is eliminate a need for trimming of the board produced, which has an advantageous effect as a cost factor per unit of board produced.
Another object of the present invention is to create a homogeneous density over the entire cross section of the board, the board being of quality construction at a reduced cost.
The forming and transporting of the mat of pressed stock takes place on the molding and charging belt in a space bounded at the longitudinal sides, two upright side supporting bands on the molding and charging belt running along therewith and guiding the pressed stock and the mat of pressed stock formed from it in such a way as to limit its width from the scattering station up to compaction in the prepress and in that the height of these side supporting bands correspond to the changing height of the press nip.
The installation for carrying out the process according to the invention consists in that two side supporting bands, forming a rectangular box for a loose fill, are arranged upright on the molding and charging belt, on both longitudinal sides, and are guided such that, from the scattering station to through the prepress, they run synchronously with the molding and charging belt. After the prepress, they run back in a circulating manner, supporting rollers bearing against the side supporting bands in the region from the scattering station up to the prepress. The side supporting bands being forcibly guided and held by supporting rollers such that they are bent with the bending plate outwardly in an L shape in the region of the prepress in a way corresponding to the respective height s of the press nip.
Further advantageous measures and refinements of the subject matter of the invention emerge from the following description with the drawings.